Electrochemical cells are constituted essentially by two materials, one of which oxidizes (the positive electrode) and the other reduces (the negative electrode). These materials react together to deliver reaction products and energy. In normal operation, the electrochemical reaction takes place via the electrolyte and the energy is recovered in the form of electricity. The temperature of a storage cell may rise locally for accidental reasons, e.g. in the event of an internal electrical short circuit. If the local temperature in a storage cell becomes too high, e.g. greater than 250.degree. C. for a lithium cell, then the materials can react with their environment in exothermal manner, at a speed that depends on temperature and pressure. In the worse case, if the heat produced cannot be evacuated quickly, then a thermal runaway phenomenon occurs and the entire storage cell is subjected to combustion on a short or very short time scale. This reaction can be very fast, producing a large quantity of gas, thereby bursting the metal envelope of the storage cell if it was not provided with a safety valve (this phenomenon is often referred to as an "explosion"). Under such circumstances, it is necessary in the event of thermal runaway occurring to ensure that the speed of reaction is as slow as possible, in particular to avoid pressure rising excessively at the reaction front, thereby making it as easy as possible for the gas to be vented via the safety valve.
Prior art solutions applied to electrochemical cells relate only to weakening the outer envelope for the purpose of enabling it to open at a determined pressure that is lower than the bursting pressure of the envelope itself.
Thus, document FR-A-2 627 327 discloses a device constituted by a V-shaped groove formed in the bottom of the container, thereby leading to a thinning of the wall. A safety valve is situated at the end (or ends) of the spiral-wound group, in the cover or the bottom of the container. In the event of the electrodes being subject to combustion halfway along the spool, the gas produced cannot be evacuated. Pressure rises strongly, thereby accelerating the chemical reactions and the combustion of the electrodes. The situation becomes catastrophic and can lead to the envelope bursting, since the vent is insufficient for evacuating all of the gas that is given off.
According to document EP-A-0 305 880 a line of weakness describing a multiple tongue is formed in the bottom of the container.
In document WO-A-82/02117, the alkaline magnesium cell described has two envelopes, the inner envelope being sealed. The outer envelope may include ribs on its inside face.
Finally, document JP-A-60 200 56 describes a cylindrical alkaline battery including a safety valve and a V-shaped rib constituting a zone of weakness.
In none of those documents is anything described that makes it easier to collect gas inside the electrochemical cell and bring it to the vicinity of the vent device. Unfortunately, particularly with cells of large size, the gases that are produced very fast during combustion at the reaction front (electrodes) cannot easily be evacuated through the spiral-wound electrode group itself, given its geometrical configuration. This applies in particular to cells of cylindrical shape (generally, but not necessarily of circular base), since the envelope is mechanically very strong and does not deform under the applied pressure, as can happen with a cell that is of prismatic shape.
It should be observed that document EP-A-0 244 261 describes an electrolytic capacitor in which the diameter of the active portion (coil) is smaller than the inside diameter of the envelope. The coil is free and is held inside the envelope by its connections only, which is not possible for a primary or a secondary cell, given the greater density of its electrodes and the constraints concerning resistance to vibration and shock. In addition, with a capacitor, there is no electrolyte and the electrodes are secured to each other by means of a resin. That is not the case of an electrochemical cell which requires an envelope to hold the electrodes in place. Finally, that document which recommends filling the container with solidified resin says nothing about providing space to allow gas to escape under such conditions.